I did it with an old guitar where the neck was starting to bend a bit, to reduce the tension. Worked fine. A mellow sound that made a nice change.

Might be more problems doing it the other way round, since the steel strings exert a good bit more pull. But I can't think how switching to nylon could in any way hurt the guitar. If there's a truss rod in the neck it would be easy enough to compensate if the neck relaxes a bit and gives him a bit of a buzz. But I'd think unless the action was set really low in the first place he'd probably have no problems, even if there's no truss rod.